The invention relates to an NMR-apparatus comprising a magnet for generating a steady uniform main magnetic field, a resonator arrangement for generating an RF magnetic field which is oriented at least substantially perpendicularly to the main magnetic field, and at least one drive mechanism for an adjusting-element which mechanism drives the adjusting element for matching the resonator to an RF generator and/or for adjusting the resonator tuning.
FIG. 1 shows a NMR apparatus of this type. It comprises an electromagnet 1 with four coils which generates a steady uniform magnetic field oriented in the same direction as the common horizontal coil axis. A patient 3 positioned on a table top in the interior of the electromagnetic is enclosed by an RF coil 4, which generates an RF pulsating field which is oriented perpendicularly to the main magnetic field generated by the electromagnet. The frequency of the RF magnetic field is proportional to the flux density of the main magnetic field, which may be between 0.1 T and 2 T, depending on the construction of the electromagnet. The proportionally constant is equal to the gyromagnetic ratio. This enables nuclear spin resonance to be obtained inside the volume enclosed by the RF coil. Four gradient coils 5, which generate a magnetic field which is oriented in the same direction as the main magnetic field and which varies linearly in this direction, ensure that this excitation is restricted to a vertical layer 6.
In such apparatuses it is important that the frequency of the RF field generated by the RF coil corresponds exactly to the nuclear spin resonance frequency, dictated by the gyromagnetic ratio and the flux density of the main magnetic field, and that the resonator comprising the RF coil 4 is always matched to the RF generator which energizes the RF coil.
However, depending on the size and region of the body of the patient to be examined, the resonance frequency may change and the quality factor of the resonator may decrease. This effect is produced as a result of the dielectric properties and the electrical conductivity of the body tissues. As a result of this, the RF generator and the RF receiver, which is subsequently connected to the resonator and which receives the spin resonance signal originating from the body region to be examined, are no longer matched to the resonator at the spin-resonance frequency.
After the patient has been positioned and before the actual NMR examination is started, it is therefore necessary to re-tune the RF generator to the predetermined spin-resonance frequency and to match the resonator to the RF generator. This is possible by means of the electrical circuit shown in FIG. 2. The resonator comprises the coil 5 as well as a capacitor 9 and a variable capacitor 7, arranged in parallel with the coil 5. In the case of a suitable dimensioning the capacitor 9 may be dispensed with. Via a variable capacitor 8, this resonator is connected to an RF generator 10. Matching is effected by means of the variable capacitor 8 and tuning is effected by means of the variable capacitor 7, the two adjustments being interdependent.
Adjusting the variable capacitors is difficult. These capacitors are arranged outside the RF coil inside the electromagnet and cannot be adjusted by means of stepping motors or the like, because the ferromagnetic parts thereof would disturb the homogeneity of the magnetic field. Therefore, the stepping motors must be arranged outside the electromagnet at a distance such that they do not affect the homogeneity of the main magnetic field and their operation is not disturbed by stray fields of the electromagnet. Therefore, the stepping motors have to be coupled to the adjusting elements inside the electromagnet via long actuating rods. It is not possible to arrange the adjustable elements outside the electromagnet, because then they would have to be connected to the resonator in the interior via electrical leads whose length (in particular in the case of high magnetic flux densities and high spin-resonance frequencies) is no longer small in comparison with the wavelength and thus would give rise to disturbances.
It is the object of the invention to construct an NMR apparatus of the type specified in the opening paragraph in such a way that the drive mechanism for adjusting the adjusting elements can be arranged inside the electromagnet without affecting the homogeneity of the main magnetic field.